In the construction of an aromatic ring such as a benzene ring, a quinoline ring, a pyridine ring, a pyrrole ring, a pyrazole ring, or a pyrimidine ring having substituent(s), a method utilizing a vinamidinium salt has been known and, currently, its usefulness has been reevaluated in many reports (see, e.g. Patent Documents 1-3 and non-Patent Documents 1-6, and 9). Among them, the method is carried out industrially as a pyridine ring construction reaction in a synthesis step of Etoricoxib which is a COX-2 inhibitor (see, non-Patent Document 6).
However, when using the vinamidinium salt industrially, there is a difficult point that isolated crystals thereof have hygroscopicity and therefore, cause degradation by deliquescing. Crystals of a halogen anion salt having a counteranion such as Cl− are said to have particularly strong hygroscopicity and therefore, it is problematic to deal with the crystals by isolating them in a larger scale production (see, non-Patent Document 12).
Then, it seems that crystals known from old, for example, crystals of ClO4− salt were used at first. However, as a result of safety evaluation, it has become apparent that the crystals have significantly explosive properties (see, non-Patent Document 7) and therefore, a BF4− salt is now used (see, Patent Document 2). However, there is a problem that the BF4− salt corrodes a glass container in case of carrying out a heating reaction.
Meanwhile, currently, a PF6− salt has newly been isolated and the use thereof has been reported (see, non-Patent Documents 6, and 8-13), but there is a drawback that the PF6− salt is more expensive than other salts.    Patent Document 1: JP-A-62-10083    Patent Document 2: JP-A-2001-139555    Patent Document 3: WO 2002/28840    non-Patent Document 1: Synthesis, (2001), (9), 1351-1355    non-Patent Document 2: Synlett, (2000), (8), 1172-1174    non-Patent Document 3: Tetrahedron Letters, (1992), 33(28), 3989-3992    non-Patent Document 4: The Journal of Organic Chemistry, (1990), 55(15), 4735-4740    non-Patent Document 5: Synthesis, (1988), 746-748    non-Patent Document 6: The Journal of Organic Chemistry, (2000), 65, 8415-8420    non-Patent Document 7: Chemical & Engineering News, (2000), 78(10), 8    non-Patent Document 8: Organic Letters, (2002), 4(17), 2969-2972    non-Patent Document 9: The Journal of Organic Chemistry, (2004), 69, 1298-1308    non-Patent Document 10: The Journal of Organic Chemistry, (2001), 66, 4194-4199    non-Patent Document 11: Organic Letters, (2000), 2(15), 2339-2341    non-Patent Document 12: The Journal of Organic Chemistry, (2000), 65, 4571-4574    non-Patent Document 13: The Journal of Organic Chemistry, (2001), 66, 251-255